1. Field of the Invention
The present invention relates generally to a joining process of aluminum alloys to titanium alloys, and in particular, to such process useful in the aerospace industry.
2. Description of Related Art
It is generally known that when manufacturing semi-finished products and structural elements for aeronautical construction, certain required properties generally cannot be optimized at the same time independently of one another. The monolithic metallic structural elements with variable properties in space are thus very much in demand in the existing context in the aeronautical industry. Structural elements are subjected to a wide variety of contradictory constraints that require particular choices about materials and working conditions that can lead to unsatisfactory compromises.
U.S. published application Ser. No. US 2005/156095 explains that for manufacturing seat mounting rails of aircraft, it is advantageous to use a material highly resistant to corrosion such as titanium alloys. However, titanium alloys are more expensive and have a higher density than aluminum alloys, which is not advantageous with regard to the constant need for cost and weight reductions in the manufacture of commercial aircrafts. It is proposed to make a seat mounting rail with a lower section made of a first material, such as high strength aluminum alloy and an upper section made of a second material different from the first material, such as a titanium alloy. The first and second materials are interconnected by a homogeneous metallurgical interconnection or bonding.
Among welding techniques, two main families may be distinguished. In fusion welding processes, such as resistance spot welding, flash butt welding, laser welding, arc welding electron-beam welding, the weld is made above the melting point, in the liquid phase. In solid state welding such as friction welding, friction stir welding, or diffusion welding, the weld is made below the melting point, in the solid phase.
Diffusion welding of titanium and aluminum has been reported in “Properties of diffusion welded hybrid joints titanium/aluminum, J. Wilden, J P Bergmann, S. Herz, Proceedings of the 3rd International Brazing and Soldering Conference, Apr. 24-26, 2006, Crowne Plaza Riverwalk Hotel, San Antonio, Tex., USA, pp 338-343)”. However, the strength of the assembly obtained is lower than 100 MPa.
Regarding fusion welding techniques, two options may be considered in order to weld an aluminum alloy to a titanium alloy: a first option is to weld at a temperature above the melting temperature of the titanium alloy in order to have fusion of both the aluminum and the titanium alloys and a second option is to weld at a temperature above the melting temperature of the aluminum alloy but below the melting temperature of the titanium alloy, this later case will be referred to herein as “weld-brazing”.
U.S. Pat. No. 4,486,647 illustrates the first option: enough welding energy is provided in order to melt the aluminum and the titanium alloys at the melt boundary. However, when the melt solidifies, titanium-aluminum compounds are produced in large quantities, resulting in a poor mechanical strength of the joint, lower than about 100 MPa.
The first option has also been reported in “Laser processing of aluminum-titanium tailored blanks, M. Kreimeyer, F. Wagner, F. Vollersten, Optics and Lasers in Engineering 43 (2005) 1021-1035”. In this article, a process is reported wherein the joining is achieved by melting the titanium base metal whilst heating the aluminum base metal through conduction. However, it appears again that a limited strength, around 200 MPa in this case, is obtained.
U.S. Pat. No. 2,761,047 provides weld brazing conditions in order to join aluminum and aluminum alloys to titanium and titanium alloys with an inert gaseous arc torch. The process disclosed comprises a cleaning step which is said to be best accomplished when the torch has a non consumable electrode and is of the ultra-high frequency alternating current type.
Laser weld brazing of aluminum and titanium without filler metal is also reported in “Investigation of Laser-Beam Joined Titanium-Aluminum Hybrid Structures, Applied Production Technology APT'07, Bremen, Sep. 17-19, 2007”. Mechanical strength is improved compared to the first option, however it is still not higher than 242 MPa for a weld between a TiAl6V4 alloy and a 6056 alloy in the T4 condition, aged after welding to the T6 condition. The reported joining speed at the conference was around 0.2 m/mn and leads to a wide heat affected zone of around 20 mm.
Weld-brazing aluminum to titanium has proven difficult, the strength of the joint may be increased and the process output improved. There is a need for an improved method capable of weld-brazing aluminum alloy parts to titanium alloy parts, with a high output capable of providing high strength welding joints.